1. Field of the Invention
The present invention relates to a nitride semiconductor crystal, a manufacturing method of a nitride semiconductor freestanding substrate and a nitride semiconductor device, and further specifically, to the nitride semiconductor crystal from which a plurality of nitride semiconductor freestanding substrates can be obtained, a manufacturing method of the nitride semiconductor freestanding substrate for manufacturing the nitride semiconductor freestanding substrate from the nitride semiconductor crystal, and the nitride semiconductor device manufactured by using the nitride semiconductor freestanding substrate.
2. Description of the Related Art
A nitride semiconductor represented by gallium nitride (GaN), aluminium gallium nitride (AlGaN), and indium gallium nitride (InGaN), is highlighted as a light emitting device material for covering a wavelength region from ultraviolet to green color, and also as an electronic device material having high temperature operation and high output operation.
In a case of a conventional semiconductor other than the nitride semiconductor, in most cases, a freestanding substrate is prepared comprising a single crystal, which is the same kind as the semiconductor, and a device structure is formed thereon by each kind of crystal growth method, to thereby realize and put to practical use various devices.
Meanwhile, in a case of the nitride semiconductor, it is difficult to obtain a single crystal freestanding substrate composed of the nitride semiconductor such as GaN and AlN, and therefore there is no other choice but to use a hetero-substrate such as sapphire and SiC. In this case, high density defect (dislocation) is generated in a nitride semiconductor layer that grows on the hetero-substrate, and this is a major factor of preventing improvement of device characteristics. If explained with a typical case as an example, a service life of a semiconductor laser depends on a dislocation density in a crystal strongly, and therefore in a nitride semiconductor element formed by a crystal growth on the hetero-substrate, it is difficult to obtain a practical service life of the element.
In recent years, a GaN single crystal freestanding substrate with low defect density has been supplied by each kind of method, and the semiconductor laser using the nitride semiconductor is finally put to practical use. Various methods are proposed as manufacturing methods of the GaN freestanding substrate. As a typical one of them, a method of growing GaN thick on a seed substrate by HVPE (Hydride Vapor Phase Epitaxy), and removing the seed substrate during growth or after growth; an Na flux method of separating GaN on the seed crystal by pressuring an entire body by nitrogen in a state that Ga metal contained in a melted Na; a high pressure synthesizing method of directly synthesizing GaN from Ga and nitrogen at high temperature and under high pressure; an ammonothermal method of dissolving Ga and GaN into ammonia and separating GaN on the seed crystal at a lower temperature and under a lower pressure than the high pressure synthesizing method, and a sublimation method of synthesizing GaN from Ga vapor and ammonia, are known.
Among above-described methods, several methods using HVPE are most successful at the present point. Then, by polishing front/rear surfaces of a freestanding single crystal of GaN manufactured by the method using HVPE, the GaN freestanding substrate with large area (diameter of 2 to 3 inches) is realized. As a method of using typical HVPE, a method of vapor-depositing Ti on the surface of a GaN thin film on a sapphire substrate, forming voids by applying heat treatment thereto, growing GaN thick thereon by HVPE, and separating the sapphire substrate from the void portion (Void-Assisted Separation Method:VAS method, see document 1); and a method of growing GaN thick on the GaAs substrate with a surface partially covered with an insulating mask by HVPE, and thereafter removing the GaAs substrate (DEEP method, see document 2), are known.    (Document 1) Yuichi OSHIMA et al., Japanese Journal of Applied Physics, Vol. 42 (2003), pp. L1-L3    (Document 2) Kensaku Motoki et al., Journal of Crystal Growth, Vol. 305 (2007), pp. 377-383.